For the operation of a circuit breaker, such as a medium voltage vacuum circuit breaker, it can be necessary to generate a high force to press a first moving electrical contact to a second corresponding fixed electrical contact. The force can be generated by a magnetic actuator. Therefore, the magnetic actuator comprises a coil for generating an electrical field, a core for forming this field and a movable plate which is attracted by the core. When attracted by the core, the movable plate generates the force used for actuating the circuit breaker.
In an open position, the movable plate can be away from the core such that a gap (which can be filled by air) is formed. The coil moves towards the movable plate and intrudes into the air gap, which can lower or even prevent the operating ability of the device. Normally, the intrusion into the gap can be avoided by one or more grooves in the coil-facing sides of the core and the flanks of the core, so that a locking piece can be interposed into these grooves. The locking piece or locking part can be a stopper or stopping means for the movement of the coil towards the gap.
EP1843375A1 shows an electromagnetic actuator for a medium voltage circuit breaker with an actuator having an electromagnet exhibiting a magnet core with a rectangular profile, and a round upper yoke corresponding to the electromagnet.
US2008272659 A1 shows an electromagnetic force driving actuator and a circuit breaker using the same.
The design with grooves and locking pieces can reduce the usable space for the coil, thus reducing the potential efficiency of the device. If the coil space is to be kept constant, the height of the core and the flanks can have to be increased, thus increasing the undesired stray flux of the magnet, and also increasing the overall dimensions of the device. Further, such grooves can increase the magnetic resistance in the core and the flanks. In this case, the grooves can disturb the distribution of the magnetic flux close to the air gap, jeopardising the flux concentration. Both actions can result in a reduced holding force.